The main problem with cancer chemotherapy is the lack of differentiation between tumor- and rapidly-dividing cells in normal tissues. This causes substantial side effects, which can in the long run lead to secondary cancers induced by the treatment. Chemotherapeutic agents often cause unwanted general cytotoxicity, and activities of several DNA repair pathways enable tumor cells to survive by removing lesions (Helleday et al., 2008).
Small-molecule inhibitors of checkpoint pathways or DNA repair machineries were identified and used as cellular radio- and chemosensitization compounds in clinical trials (Bolderson et al., 2009). Given the differences in checkpoint and DNA repair alteration during tumorigenesis, the therapeutic efficacies of these strategies were found various depending on the checkpoint context of tumor (Jackson and Bartek, 2009; Jiang et al., 2009).
Moreover, derangement of DNA damage response could cause the accumulation of DNA error during therapy, which might provoke secondary tumor development (Mimeault et al., 2008). Therefore, it is important to develop a chemosensitization regimen that does not disrupt the checkpoint network while specifically inducing cancer cell death with little side effect.
An important process in DNA repair is the supply of balanced and sufficient quantities of four dNTPs (Niida et al., 2010b). Ribonucleotide reductase (RNR)-mediated reduction generates not only dADP, dGDP and dCDP but also dUDP, directly from the corresponding NDPs (Nordlund and Reichard, 2006). RNR is composed of R1 and R2 subunits, of which the level of R2 is cell-cycle-regulated (Bjorklund et al., 1992; Engstrom et al., 1985) and often elevated in tumor cells (Jensen et al., 1994; Zhang et al., 2009). It has been reported that R2 overexpression confers oncogenic potential (Fan et al., 1998). An analogue of R2, p53R2, can substitute for R2 to form RNR enzyme, and its function is important for DNA repair in quiescent cells (Hakansson et al., 2006; Pontarin et al., 2011). Nucleotide diphosphate kinase converts all these dNDPs to dNTPs, which include dUTP (Mathews, 2006; Reichard, 1988). Pyrophosphorolysis of dUTP by dUTPase or deamination of dCMP forms dUMP, which is converted to dTMP by thymidylate synthase (TS) (Mathews, 2006; Reichard, 1988). The action of thymidine kinase (TK) also generates dTMP from thymidine (Amer and Eriksson, 1995). Thymidylate kinase (TMPK) subsequently catalyzes the formation of dTDP (Ostermann et al., 2000; Reichard, 1988). Thus, dTDP is the only dNDP, the formation of which cannot be directly derived from RNR reaction.
Conventional anti-cancer therapies often directly induce genotoxicity (Garg et al., 2010). For example, thymidylate synthase (TS) inhibitor, 5-FU or 5-FdUrd, blocks the conversion of dUMP to dTMP, causing dUTP to accumulate and 5-FdUTP formation (Longley et al., 2003). Since DNA polymerases cannot discriminate between dUTP and dTTP (Bessman et al., 1958; Mosbaugh, 1988), excessive amounts of dUTP and 5FdUTP are mis-incorporated into DNA, triggering DNA damage-induced cell death (Ahmad et al., 1998). Consequently, such anti-metabolites produce excessive DNA damage due to erroneous nucleotide incorporation and causes cancer cells death while being highly toxic to normal cycling cells (Ahmad et al., 1998).
It is known that double-strand breaks (DSBs) in proliferating cells are mainly repaired by homologous recombination (HR) in which a single DSB needs more than 10 thousands of dNTPs new incorporation (Robert et al., 2011; San Filippo et al., 2008). As such, RNR function in supply of dNTPs is critical for HR repair (Burkhalter et al., 2009). Of note, blocking RNR on its own induces DNA damage signal and replication stress (Helleday et al., 2008). Since dTDP formation specifically requires TMPK function, we propose that blocking TMPK may decrease the efficiency of DSBs repair and sensitize tumor cells to genotoxic insults.
Accordingly, there is a need to develop novel therapeutics than can induce cancer cell death, when used alone or in combination with other anti-cancer therapies, while reducing the toxic effects caused by such treatments.